In commercial vehicles, brake pressure control which limits the brake pressure (electronic brake force limiter, EBL) is used, in particular, for distributing the braking effect desired by a braking system or a driver to the vehicle axles in a suitable way, and so the vehicle is decelerated according to a driver's intent, which is predetermined by a driver, or according to a setpoint vehicle deceleration and simultaneously remains stable according to the driving situation. This is the case because, in the event of strong braking, i.e. in the event of a high setpoint vehicle deceleration, the vehicle weight is displaced onto the front axle, and so rear wheels on the rear axle are pressed less strongly onto the underlying surface, whereby a lockup of the rear wheels is supported.
By limiting the brake pressure at wheel brakes of the rear wheels, a premature lockup of the rear wheels can be avoided when braking is carried out. The brake pressure can be limited according to an axle load, since, in the case of heavily loaded vehicles, more braking force can be applied onto the road via the rear axle than is the case with unloaded vehicles, in which a braking of the rear wheels that is too strong further amplifies the above-described effect; the tendency of the rear wheels to lock up increases.
A mechanical displacement sensor can be provided for measuring the load of leaf-sprung vehicles; the mechanical displacement sensor is deflected to a greater or lesser extent depending on the load. In air-sprung vehicles, a pressure sensor that measures the pressure in a support bellows can be utilized for this purpose. Depending on the load-dependent measured signal, the weight can be inferred and, depending thereon, a limitation of the brake pressure can be carried out.
In vehicles without such a mechanical displacement sensor or pressure sensor, in which a direct measurement of the load is not provided, an electronic brake pressure limitation can be provided. In this case, a control of the brake pressure is carried out at the wheel brakes of the rear wheels as a function of a difference between a wheel speed of the front wheels and the wheel speed of the rear wheels or as a function of the difference between a brake slip of the front wheels and the brake slip of the rear wheels. Since the difference in the wheel speeds or the brake slips is load-dependent, the brake pressure is correspondingly distributed onto the vehicle axles, in order to prevent the wheels of the rear axle from prematurely exhibiting a tendency to lock up and in order to decelerate the vehicle as desired.
DE 10 2008 009 948 B3 discloses an electronic brake pressure control in which the brake pressure at the wheel brakes is controlled depending on an axle load sensor and the setpoint vehicle deceleration. In the event of failure of the axle load sensor, the brake pressure is controlled as a function of the difference of the brake slips at the vehicle axles, i.e. as the function of a measured actual differential slip.
DE 10 2011 118 130 B4 also discloses an electronic brake pressure control in which the braking forces are distributed onto the vehicle axles depending on an ascertained actual differential slip or an ascertained differential speed of the wheels, in order to decelerate the vehicle as desired. In this case, the brake pressure at the wheel brakes of the rear wheels is limited if the actual differential slip exceeds a setpoint differential slip. The setpoint differential slip is dependent on the setpoint vehicle deceleration in this case, wherein the setpoint differential slip increases at higher setpoint vehicle decelerations in order to ensure a more sensitive transition to an ABS control which engages sooner in this case than in the event of a constant setpoint brake slip. The disadvantage in this case is that an increase of the setpoint differential slip at a high vehicle setpoint deceleration effectuates an early activation of the ABS control, whereby the braking behavior becomes less calm and less safe, since the ABS control must engage sooner.
An electronic brake force distribution is disclosed in DE 10 2006 045 317 A1, in the case of which the braking forces are distributed onto the vehicle axles depending on the vehicle deceleration rather than depending on the differential slip. For this purpose, it is provided to measure an actual vehicle deceleration and, once a maximum deceleration, i.e. a certain threshold value for the actual vehicle deceleration, has been reached, to control the brake pressure at the rear axle. The maximum deceleration is defined, in this case, in such a way that the rear wheels have a greater tendency to lock up at this value than do the front wheels. A tendency for the rear wheels to lock up is therefore adapted to the tendency for the front wheels to lock up. After the maximum deceleration has been exceeded, it is provided to build up the brake pressure at the wheel brakes of the rear wheels in a controlled manner, wherein the gradient is lower than the gradient of the brake pressure that is building up at the wheel brakes of the front wheels. As a result, the brake pressure at the wheel brakes of the rear axle tracks the brake pressure at the wheel brakes of the front wheels, i.e. is held lower by a certain value. The disadvantage in this case is that, above the maximum deceleration, the braking behavior is turbulent overall, due to the tracking, and is perceived as unpleasant.